This invention relates to an apparatus for supervising an incoming call to a local station sent from other station, and more particularly, to a trunk apparatus for a loop dialing trunk line.
A conventional interoffice incoming call uses a loop dialing type transmission trunk, i.e., a loop dialing type relay line trunk.
As described in detail later, this is achieved by forming a loop on the transmitting station side with the trunk of the receiving station side which supervises the loop and determines the incoming call by an existence of a loop detection.
FIG. 1 shows a prior art loop dialing type transmission line or trunk line.
A power source 1 of -48V is connected from a power supply supervisory circuit 2 and forward direction diodes 3 and 4 via relay contact points 5a, 5b, 8a and 8b to a tip line and a ring line on the line side. A sending control circuit 6 is also connected to the line side via relay contact points 5b, 8a and 8b.
A voice signal is connected to a transformer 11, usually via a close relay contact point 7a and a capacitor 9 for a DC cut. It is further separated into a receiving signal and a sending signal via a double line/quadruple line conversion circuit 12 and connected to the network side of a private branch exchange (hereafter abbreviated as PBX).
13 is an overvoltage protection circuit. When relay contact points 8a and 8b act in a receiving mode and the polarity between the tip line and the ring line is inverted as a receiver response to the incoming call, from another station relay contact points 8a and 8b are switched in the opposite direction to that shown in the figure. A loopback detection signal 10 detected by the power supply control supervisory circuit 2 is sent to a control part not shown in FIG. 1. Here a receiving control is performed, which controls the sending control circuit 6 via 1 of FIG. 1.
FIG. 2 shows an equivalent circuit connected to the loop dialing type transmission line trunk of the above configuration. As FIG. 2 shows, between station A (PBX A) and station B(PBX B), the equivalent circuit is arranged so that the polarities of the power sources 1 in PBX-S A and B are set to face each other.
Where both PBS-S or stations supervise an incoming call, their relay contact points 5a and 5b are connected to the diode 3 and 4. Thus, in an incoming call supervision condition (which is set to detect a call from the other PBX to present PBX); a current does not flow in the part equivalent to the power supply control supervision circuit 2 in FIG. 1. Here, when the voltages of the power sources 1 of the two stations are different, diodes 3 and 4 to prevent flowing current in the reverse direction.
Next, when for instance station (PBX) B sends a call to station (PBX) A, relay contact points 5a and 5b of the station (PBX) B side are switched to the sending control circuit 6 side to form a loop with the direct current power source 1 of the station (PBX) A side, as shown in FIG. 2. Therefore, loop currents I.sub.0 and I.sub.1 flow in the part equivalent to the power supply control supervisory circuit 2 in FIG. 1. The power supply control supervisory circuit 2 can detect these loop currents, and station (PBX) A can detect a sending action of station (PBX) B.
In the receiving supervisory method using loop dialing, as shown in FIG. 3, a loop dialing type transmission line trunk 14 of FIG. 1 is set between a line 16 and, for instance, a network 15. Yet, due various factors, the line 16 is superposed with a noise N called a common mode noise signal as shown in FIG. 3 between it and the ground G.
In the existing loop dialing type transmission line trunk of FIG. 1, the impedance of the tip line and the ring line against the ground is kept in an equilibrium condition. (Namely, earth balance is maintained.) In a communication condition, loop currents I.sub.0 and I.sub.1 flow in the normal direction of diodes 3 and 4 of FIG. 1.
As so far described, when the earth balance of the tip line and the ring line is well maintained, said common mode noise N is similarly superposed on the loop current I.sub.0 of the tip line side (a positive current) and to the loop current I.sub.1 of the ring line side (a negative current). Therefore, as shown in FIG. 4, since (I.sub.0 -I.sub.1).sub.AC, namely, the alternate current portion of the differential between I.sub.0 and I.sub.1 does not arise, the common mode noise N is not converted to the noise between the tip line and the ring line (a normal mode noise). Consequently, noise between the tip line and the ring line (the normal mode noise) is suppressed.
Here, the loop current I.sub.1 is the current flowing from the opposite station or PBX via the ring line, and is shown as negative current when the current I.sub.0 flowing to the opposite station is defined as positive current, and it does not mean the reverse direction from the arrow of FIG. 1.
However, in this prior art, when the line 16 of FIG. 3 is very long and a commercial power line is set very close to a tip-ring line over a long distance, and when the common mode noise N of a large amplitude that biases in the reverse direction of the diodes 3 and 4 of FIG. 1 because a noise of a commercial alternate current power source is superposed, as shown in FIG. 5, a current clips around a 0 level, and the loop currents I.sub.0 and I.sub.1 become different from each other. Thus, a difference in the alternate current portion between the loop currents I.sub.0 and I.sub.1 arises. This differential causes a normal mode noise to be generated. That is, the existing loop dialing type transmission line trunk of FIG. 1 can secure an earth balance sufficient for the common mode noise N that turns the diodes 3 and 4 on, whereas it has the problem that earth balance is lost when the large common mode noise N occurs, causing diodes 3 and 4 to be turned off.